THE dozens of reporters, producers and writers who launched “Inside Edition” in January 1989 are gathering to mark the tabloid show’s 20th anniversary tomorrow at Caffe Cielo on Eighth Avenue. “It’s the last tabloid [show] standing,” said one organizer. “In January of 1989, Ronald Reagan was president, cellphones weighed 10 pounds and Bill O’Reilly was an obscure ABC reporter. Boy, did we have it good.” Invites specify “Free food – cash bar,” but one producer laughed, “It will be fun buying O’Reilly a drink now that he’s making $15 million a year.” Also expected: Rolonda Watts, Deborah Norville, Nancy Glass and the producers, led by John Tomlin.